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The verses:

“发汗后，水药不得入口为逆，
若更发汗，必吐下不止。
发汗吐下后，虚烦不得眠
若剧者，必反复颠倒，心中懊憹，
栀子豉汤主之；” (line 76a)

“若少气者，栀子甘草豉汤主之；
若呕者，栀子生姜豉汤主之。” (line 76b)

“发汗若下之，而烦热胸中窒者，栀子豉汤主之。” (line 77)

“伤寒五六日，大下之后，身热不去，心中结痛者，未欲解也，栀子豉汤主之。” (line 78)

The Big Picture:

In a taiyang disease (think of it as some kind of exogenous pathogen causing an immune reaction), perspiration is usually used. However, for some reason or other, the person is unable to take in medicine (a stomach reaction) and vomits it out. We know from line 74 that this is called 水逆. Here, the situation is similar. In both cases, the taiyang disease or the accompanying perspiration causes dehydration. The person is not only vexed (烦), he is also very thirsty. We learn that this is due to “胃中干” (see Line 71a) and requires just drinking water slowly for mild cases. We also know that 五苓散 (see line 72b) if indicated if 脉浮 and 小便不利 are added to the list of symptoms.

The biggest mistake now is to continue treatment using perspiration, which is what we see being carried out in line 76a. This WILL result in vomiting and diarrhea. After which, the patient will experience 虚烦不得眠 or if more sever, 反复颠倒，心中懊憹.

Line 77 describes another crazy treatment done by the doctor:

• he doesn’t try to treat the 水逆 by further perspiration (which was done in line 76a). He induces precipitation. The end result is 烦热胸中窒, i.e. the patient is vexed and and feel strangulated in the chest area.
• fortunately, this doctor never tried to treat by inducing vomit. That would have been hazardous!

Line 78 is slightly different from the above two cases. It’s not about perspiration causing 水逆. Rather, it’s a case of taiyang disease lasting six to seven days, where the doctor decides to induce precipitation. The result is hazardous: 身热不去，心中结痛, i.e. the body heat (fever) doesn’t go (OBVIOUSLY!), and there is a binding pain in the chest. I think this is worse than the line 77 case, because that I associate more with respiration, and this more with the heart itself.
The commonality:

The commonality in this all is that 栀子豉汤 is used to treat the resulting problems of a vacuous body accompanied by vexation (if mild) and pain in the chest area (if more severe).

Why 栀子豉汤？

We know that:

1. the heat is affected in the mind and also the chest area
2. it’s heat that may be due to a vacuity (deficiency) from the perspiration/vomiting/precipitation

This heat can be thought of 郁热. The method is to 清宣郁热. For this, gardenia (栀子) is suitable. As for 香豉, it can resolve the exterior in a very light manner, perhaps doing what was not done earlier when perspiration was implemented overly aggressively.

Whatever the textbook explanation, I think of this decoction as a 解表剂. It takes away the fever (heat), albeit using what is commonly recognized as a 清热泻火 herbs rather than a 解表 herb. It also resolves the exterior in a very light manner, causing a light sweat, without even needing to use 桂枝.